I really did want to like this book. The author is a good storyteller, does a wonderful job of evoking a real sense of the period (no shirking of historical research here), and the idea of a female pope is a fascinating one. One can imagine that spiritual women denied access to the church, or clever women denied access to learning, might indeed have sought to escape the confines of their gender.
But somewhere between Joan/John outarguing Greek philosophers, becoming a famous healer, inventing intinction, miraculously surviving beatings/viking raids/plague, inventing modern courtroom procedure (witnesses, questioning), establishing orphanages, saving peasants from floods, cleverly applying her knowledge of hydraulic engineering to save the Vatican from an invading Frankish army, saving the pope from assassination, exposing ecclesiastical corruption, and thwarting a raging city fire, I found it harder and harder to keep suspending disbelief. This Pope Joan is a liberal, feminist, secular humanist, Dark Ages superhero rather than a living, breathing, believable woman of her time. The author takes such pains to eliminate anachronism in all other aspects of the novel: perhaps that is why John/Joan's highly anachronistic behavior & beliefs seem so grating in contrast.
John/Joan's enamorata Gerold is also a disappointment. There is no attempt at character development here. Think Ken to Joan's Barbie, Ned to Joan's Nancy Drew ... the tall, lusty, handsome, resourceful hero of any one of a thousand cheesy romance novels.
Finally, I was disappointed by the author's overreliance on deus ex machina. Far too often she relies on improbable plot twists, timely intercessions and amazing coincidences to move her plot forward. I don't want to spoil the plot for potential readers - but I will say that Joan always seems to be behind the right wall when there is a useful conversation to be overheard, Viking raids have never been more conveniently timed, and old friends/allies have a way of miraculously appearing just when they are most needed.
I guess I'm saying that while this is an entertaining book, it is certainly not a great book. Be prepared to enjoy it for the story & the history, but not necessarily for the literary merit.